The present invention relates to an angular velocity sensor.
As well known, angular velocity sensors are used for various objects such as motion control of a vehicle or an airplane and control for preventing hand shaking from affecting a video camera. As one of the angular velocity sensors, an angular velocity sensor which employs an electrostatic drive system for inducing drive vibration by using electrostatic attraction generated between electrodes is known. As an example of a conventional angular velocity sensor which employs the electrostatic drive system, the plan view and the sectional view of an angular velocity sensor disclosed in U.S. Pat. No. 5,408,877 are shown in FIGS. 9 and 10. The operation principle of the angular velocity sensor 80 is as follows. In a drive gimbal plate 82 having an inertia mass member 81 fixed to the center of the drive gimbal plate 82, when drive voltages of which the phases are reversed are applied by a drive electric circuit 88 to respective drive electrodes 83 held parallel to each other on a semiconductor substrate 90 and opposing the drive gimbal plate 82, a torsional vibration about a Y-axis occurs about a torsional shaft constituted by a first torsion beam 84 by electrostatic attraction generated between the drive gimbal plate 82 and the respective drive electrodes 83. In this manner, the center of mass of the inertia mass member 81 performs simple harmonic motion in an X-axis direction.
On the other hand, when the entire system of the angular velocity sensor 80 is rotated about a Z-axis, Coriolis force F is generated in the direction of a Y-axis, perpendicular to the X-axis direction and the Z-axis direction:
F=2vMxcexa9
where v is a velocity of the center of mass in the X-axis direction, xcexa9 is an angular velocity of rotation, and M is a mass of the inertia mass member 81.
In response to the generation of the Coriolis force F, in a detection gimbal frame 85 arranged outside the drive gimbal plate 82, torsional simple harmonic motion about the Y-axis is induced about the torsion axis of a second torsion beam 86. By the torsional vibration about the Y-axis, an electrostatic capacitance between a detection electrode 87 arranged on the detection gimbal frame 85 and the detection gimbal frame 85 changes. The change in electrostatic capacitance is detected by a detection electric circuit 89 and electrically converted into a voltage, so that a sensor output voltage, which is in proportion to the angular velocity xcexa9, can be obtained.
As described above, in the prior art, since electrostatic attraction generated between the parallel plates constituted by the drive electrodes 83 and the drive gimbal plate 82 is used as a means for inducing torsional vibration, drive displacement which can be stably vibrated is regulated for the following reason. That is, since the electrostatic attraction is in inverse proportion to the square of the gap between the drive electrode 83 and the drive gimbal plate 82, when a displacement amplitude is larger than a certain threshold value, the electrostatic attraction exceeds the resilience of a torsion spring, and a pull-in phenomenon in which the plate 82 is stuck to the opposing drive electrodes 83 occurs.
Therefore, in the angular velocity sensor 80 which employs the electrostatic drive system, the maximum value of a drive displacement amplitude which can be controlled by a gap, is disadvantageously equal to or smaller than at least the gap. For the sake of convenience, in consideration of a case in which the frequency of the torsional vibration is fixed, an improvement in sensitivity of the sensor copes with an increase of the velocity of the center of mass in the X-axis direction and also copes with an increase in displacement amplitude. However, when a gap between the parallel-plate electrodes is present, the maximum amplitude is equal to or smaller than the gap (about ⅓ of the gap because of the pull-in phenomenon). Accordingly, in order to improve the sensitivity of the sensor, a means for increasing the gap is inevitably used. With the increase of the gap, the drive voltage and the opposite area must be large.
In this manner, in the angular velocity sensor which employs a conventional electrostatic drive system, sensor design for improving the sensitivity of the sensor is regulated, and an obstruction to the design occurs.
The present invention has been made in consideration of the above technical problems, and has as its object to provide a sensitive angular velocity sensor without increasing a gap between electrodes for inducing torsional vibrations.
In an aspect of the present invention, there is provided an angular velocity sensor which employs an electrostatic drive system for inducing drive vibration by using inter-electrode electrostatic attraction comprising a pair of inertial mass members which are connected to each other through a connection frame having a predetermined length such that the inertial mass members are point-symmetrical with respect to a central point of the connection frame and which are held along a plane direction of a substrate, a pair of elastic beams which are connected to each other at a predetermined interval in opposite directions along a longitudinal direction from the central point of the frame on both the sides of the connection frame, a pair of drive plates which are connected to the other ends of the elastic beams, which have an electrode constituting an electrostatic attraction generating structure together with a drive electrode arranged on the substrate, and which are supported such that the drive plates can be moved in at least one direction along the plane direction of the substrate to torsionally vibrate the inertial mass members about the central point of the connection frame, an electrode for detecting displacement of the inertial mass members in a direction vertical to the plane direction of the substrate during displacement vibration of the inertial mass members according to an inertial force generated with the torsional vibrations of the inertial mass members in a direction vertical to the plane direction of the substrate, and torsional vibration detector for detecting amplitudes of torsional vibrations of the inertial mass members about the central point of the connection frame.
Said electrode and the drive electrode in the drive plate constituting the electrostatic attraction generating structure may be parallel-plate electrodes. Said electrode and the drive electrode in the drive plate constituting the electrostatic attraction generating structure may be comb-like electrodes.
As the torsional vibration detector, a plate-like electrode opposing at least parts of the inertial mass members may be arranged on the substrate to constitute parallel-plate electrodes together with the inertial mass members. As the torsional vibration detector, a comb-like electrode may be arranged integrally with the inertial mass members, and a comb-like electrode which can opposite to the above comb-like electrode to constitute a bush structure may be arranged on the substrate.
A support member for holding the inertial mass members above the substrate along the plane direction of the substrate may be arranged integrally with the inertial mass members.